My eyes opened to that about 15 years ago in a single act of revelation. It took all of about 30 seconds. Almost all of it to gather the background information and a split second to put two and two together.

It took place in the corridor of a company I was visiting for a job interview. I was sitting on a bench waiting to be called for the next stage of the interview when two guys in suits, presumably working for this company, were passing by and, ignoring me, discussed some business matters. One of them mention in passing, like it was a normal, everyday thing, that they bought a database of searches about their company from Google and then adjusted some wording on their website according to the results. For example, because "free" came up in the search queries 10x more often than "evaluation", so they changed "download our evaluation software" to "download our free software".

Two things hit me over the head at the time:

1. "Free" is not the same as "evaluation", so what they did was, IMO, cheating.
2. But even more crucially, Google makes money by selling what people are searching for, thus enabling this kind of cheating. What's more, what you are buying can be tailored to your wishes, as in this example when they bought a search database filtered by certain criteria. If this kind of filtering is possible, what other things Google gathers about you to do other kind of preprocessing?

Those two points flashed through my head in a split second and I had my lightbulb moment. Before that, I was just an ordinary sheep like everyone else. I treated Google like everyone else - like the Best Search Engine Ever™, not giving a second thought about what is really going on.